Speaking on ABC's This Week program, Johnson said the Islamic State group's influence is spreading far beyond the battlegrounds of the Middle East.

"We're very definitely in a new environment, because of ISIL's (Islamic State group's) effective use of social media, the Internet, which has the ability to reach into the homeland and possibly inspire others," he said. "We're very definitely in a new phase in the global terrorist threat, where the so-called lone wolf could strike at any moment."

The U.S. security chief urged Americans to be vigilant and aware, but not to avoid going to public events.

FBI Director James Comey last week said authorities are concerned about IS encouraging attacks on "uniformed military and law enforcement" in online propaganda. There are "hundreds, maybe thousands" of people in the United States who have received recruitment messages from the jihadists, Comey said.

On Friday, the U.S. military bolstered security at its bases across the country.

"Because of the use of the Internet, we could have little or no notice in advance of an independent actor attempting to strike. And so that's why law enforcement at the local level needs to be ever more vigilant and we are constantly reminding them to do that," Johnson said.

He also urged leaders in the broader Islamic community to step up to counter extremism.

"It has to come from within the community. It has to come from Islamic leaders, who frankly can talk the language better than the federal government can," Johnson said. "Islam is about peace and brotherhood. There are 1.6 billion Muslims in this world. And the true Islamic faith has nothing to do with what ISIL represents."